1. Field of the Invention
This invention relates to a levitating and melting apparatus in which a conductive material to be melted is placed in an alternating magnetic field to be subjected to induction heating by causing electromagnetic induction therein, and the magnetic field is distributed in a predetermined manner to exert buoyancy due to an electromagnetic force on the material to be melted so that the material to be melted is melted under a levitating state, thereby obtaining a high purity material. The invention relates also to a method of operating the levitating and melting apparatus.
2. Description of the Related Art
A levitating and melting apparatus is an apparatus in which a material to be melted is placed in an alternating magnetic field produced in a predetermined distribution, and both induction heating and buoyancy due to an electromagnetic force are simultaneously exerted on the material so that the material is melted under a state where the material floats to be prevented from making contact with other articles such as a crucible, whereby a product of a given quality and dimensions can be obtained. The apparatus has features such as that the material does not make contact with another article in a melting process and therefore the material is hardly contaminated by foreign substances, that even a material of a high melting point can be melted, and that the loss of heat conduction is small. Because of these features, such an apparatus is used in a process of melting a material which has a high melting point and is requested to have a high purity, such as titanium, or silicon.
FIG. 8 is a perspective longitudinal sectional view showing the whole of a levitating and melting apparatus which is in an operation state, and FIG. 9 is a perspective longitudinal sectional view showing main portions of FIG. 8 in an initial operation state. These figures are shown in U.S. patent application Ser. No. 08/067,149. In the figures, the levitating and melting apparatus comprises: a crucible 1 consisting of an upper crucible 11 and a lower crucible 12; an induction coil 2 wound on the outer face of the crucible 1; a continuous charging device 3 which continuously charges chips 53 to be used as a conductive material to be melted 5 through an upper opening of the crucible 1; a control device 31 which controls the continuous charging device; a molten metal thermometer 32 for obtaining control information for the control device; a first driving device 4 which vertically moves the lower crucible 12; a first control device 41 which controls the first driving device; and a molten metal level gauge 42 for obtaining control information for the first control device. The driving device 4 and control device 41 are identified by providing the term "first" because another driving device and control device are used in the invention and these devices are required to be distinguished from each other.
The induction coil 2 consists of induction coils 21 and 22 respectively connected to AC power sources 23 and 24 for energizing the respective coils. The continuous charging device 3 has an induction coil 33 which is energized by an AC power source 34 to previously heat the chips 53. The configuration in which the induction coil 2 divided into two coils are respectively energized by the different AC power sources 23 and 24 is employed in the case where the functions are allotted among the two coils, namely, the execution of induction heating is mainly allotted to the upper induction coil 21 and the generation of flotation to the lower induction coil 22, so that the functions are efficiently executed. In the case, generally, the induction coil 22 is energized by a frequency lower than that for the induction coil 21. In the invention, however, it is not required to distinguish the two induction coils 21 and 22 from each other. Therefore, these induction coils are collectively dealt as the induction coil 2 in the following description.
As shown in the figures, the upper and lower crucibles 11 and 12 are configured in such a manner that plural segments 111 and 121 each having a predetermined shape are arranged with interposing an insulating material such as mica between them. The crucible 1 which is a combination of the upper and lower crucibles is formed into a substantially cylindrical shape having a bottom. Each of the segments 111 and 121 is made of copper and provided with cooling holes so as to be cooled by cooling water.
FIG. 8 shows a state which is near the final stage of the melting process, and FIG. 9 shows an initial state wherein a small material to be melted 5 floats on a molten metal. In other words, FIG. 8 shows a state wherein the material to be melted 5 grows as a result of processes which will be described in detail, to increase its length. Next, processes in which the material to be melted 5 is actually melted and a predetermined product is obtained will be described.
(1) As shown in FIG. 9, initially, a small amount of the material to be melted 5 is charged and the induction coil 2 is energized. This produces an alternating magnetic field in the space surrounded by the induction coil 2, and eddy currents are induced by electromagnetic induction to flow in the segments 111 and 121 and the material to be melted 5. The magnetic fluxes are distributed so as to be along the inner face of the crucible 1. Since the segments 121 of the lower crucible 12 are shaped so that the lower portion of the inner space of the lower crucible 12 is narrowed as illustrated, the magnetic flux distribution in the vicinity of the bottom where the material to be melted 5 exists has a shape which expands upward. When such eddy currents flow, the material to be melted 5 is heated. On the other hand, interaction between the eddy currents and the above-mentioned magnetic flux distribution generates an electromagnetic force which acts on the material to be melted 5 in the direction opposite to the gravity or in the upward direction. The detailed description of the generation of the force is omitted. As shown in the figures, the shape of the bottom of the lower crucible 12 is set so as to obtain a magnetic flux distribution suitable for generating the flotation. PA1 (2) The electromagnetic force starts to be exerted on the material to be melted 5 at the same time as the energization of the induction coil 2, and the material to be melted 5 starts in a short time delay to float and stops at a position where the electromagnetic force balances with the gravity. The material to be melted 5 has a high melting point and requires a considerably long period to be melted. When the temperature of the material to be melted reaches the melting point, therefore, the material to be melted has already entered the levitating state. Consequently, the material to be melted 5 is prevented from making contact with another articles, and hence free from contamination with impurities. PA1 (3) The chips 53 of the material to be melted 5 are charged by the continuous charging device 3. The chips 53 are previously heated by the electromagnetic induction heating due to the induction coil 33, to a high temperature which is lower than the melting point. The charged chips 53 then make contact with the material to be melted 5 and are heated by means of heat conduction to a temperature higher than the melting pint, resulting in that the chips is melted to be literally united with the material to be melted 5 into one body. As the continuous charge of the chips 53 proceeds, the material to be melted grows to increase the dimensions. The charging frequency is adequately controlled in such a manner that the charging of the chips 53 is conducted when the molten metal thermometer 32 indicates a temperature higher than a given value and is not conducted when the thermometer indicates a temperature lower than the given value. PA1 (4) Since the degree of increase in flotation of the material to be melted 5 is smaller than that in weight of the material to be melted, the levitating position is gradually lowered as the material to be melted 5 grows, and the lower portion of the material to be melted 5 finally makes contact with the bottom of the lower crucible 12. Since the lower crucible 12 is cooled as described above to be maintained at a low temperature in the vicinity of ordinary temperature, the portion in contact with the lower crucible is immediately solidified. In this way, a solidified portion 52 is first formed, and then grows as the material to be melted 5 grows. A melting zone 51 always exists on the top portion of the material to be melted 5, and the chips 53 are dropped into the melting zone 51. Since the melting zone 51 is on the solidified portion 52, the melting zone is prevented from making contact with the crucible 1. Consequently, the material to be melted 5 can grow to a large extent under conditions that the material to be melted is free from contamination with impurities. PA1 (5) When the growth of the material to be melted 5 proceeds to some extent, the lower crucible 12 is controlled to be moved downward so that the melting zone 51 is maintained at a predetermined position with respect to the upper crucible 11 and the induction coil 2. In this control, the position of the upper face of the material to be melted 5 is measured by means of the molten metal level gauge 42, the measuring result is supplied to the first control device 41, and the lower crucible 12 is moved by the first driving device 4 on the basis of the measuring result. PA1 (6) When the length of the material to be melted 5 reaches a given value, the operations of moving the lower crucible 12, charging the chips 53, and energizing the induction coil 2 are stopped. Since the whole of the material to be melted 5 which has grown into a cylindrical shape as shown in FIG. 8 is solidified, the solidified material is then removed as a desired product from the crucible. The dimensions, particularly the length, of the product depend on the moving distance of the lower crucible 12. Consequently, the levitating and melting apparatus has a feature that a product which is far longer as compared with the capacity of the crucible 1 can be obtained. PA1 a) introducing a small amount of the material to be melted into the crucible, and energizing the induction coil under a state where the upper and lower crucibles are close to each other and the induction coil is disposed in an outer face side of the lower crucible; PA1 b) continuously charging chips of the conductive material to be melted through the upper portion of the crucible; PA1 c) relatively moving a position of the induction coil as the material to be melted grows to increase the height in accordance with the charging of chips, to maintain the induction coil located at an adequate position with respect to a position of a melting zone of a top portion of the material to be melted; PA1 d) when the melting zone grows to reach an upper limit in the upper crucible, fixing a relative position of the upper crucible and the induction coil, and downward moving only the lower crucible, to keep the upper crucible and the induction coil located at adequate positions with respect to the position of the melting zone; PA1 e) when the lower crucible is moved by a predetermined distance, stopping the movement of the lower crucible, and stopping the energization of the induction coil; and PA1 f) removing the cylindrical material to be melted as a product from the crucible. PA1 a) Under a state where the upper and lower crucibles are close to each other and the induction coil is disposed in the outer face side of the lower crucible, a small amount of the material to be melted is introduced into the crucible and the induction coil is energized. This causes the material to be melted to be subjected to an induction heating and the temperature of the material is raised. Also, an upward electromagnetic force in accordance with the magnetic flux distribution in the crucible acts on the material to be melted so that the material floats against the gravity to suspend at a fixed position. Since the induction coil is in the outer face side of the lower crucible in which the material to be melted is contained, the induction heating and the application of flotation are conducted efficiently. PA1 b) Chips of the material to be melted are continuously charged through the upper portion of the crucible. The chips enters the material to be melted in a molten state and is then heated to melt to be united with the material to be melted, thereby increasing the size of the material to be melted. Since the increase of the flotation is smaller in degree than that of the weight, the levitating position of the material to be melted is gradually lowered as the material grows, and the material to be melted finally makes contact with the bottom of the crucible to be locally cooled and solidified. The solidified portion further grows as the material to be melted grows, and only the top portion of the material to be melted is melted to form the melting zone. PA1 c) The induction coil is relatively moved as the material to be melted further grows to increase the height in accordance with the charging of chips, so that the induction coil is maintained at an adequate position with respect to the position of the melting zone of the top portion of the material to be melted. This allows the material to be melted to stably grow irrespective of the movement of the melting zone. PA1 d) When the melting zone reaches the upper limit in the upper crucible, the relative position of the upper crucible and the induction coil is fixed, and the lower crucible is downward moved so as to maintain the positional relationship between these components and the melting zone. In the same manner as the above paragraph, this allows the material to be melted to stably grow. PA1 e) When the lower crucible is moved by a predetermined distance, the movement of the lower crucible and the energization of the induction coil are stopped. Then, the material to be melted is not heated and is subjected only to the cooling, and hence also the melting zone is solidified to obtain a cylindrical product. PA1 f) The material to be melted is removed as a product from the crucible, and the operation of the apparatus is stopped.
In FIG. 8, the gap between the solidified portion 52 of the material to be melted 5 and the inner face of the crucible 1 is illustrated so as to have a fairly large size. As seen from the above description, however, the fact is that the gap between the solidified portion 52 and the inner face of the crucible 1 is substantially zero or has a very small size. In the figure, the melting zone 51 is illustrated so as to have an irregular face. This intends to show actual phenomena such as deformation caused by vibration of the melting zone 51 at the instant the chips 53 enter the melting zone 51. When the effect of the charging of the chips 53 is not exerted, the actual shape of the melting zone 51 is maintained to be a stable axially symmetrical shape as described later.
A very large current of several thousands amperes flows through the induction coil 2, and the frequency of the current is very high or as high as several kilohertz. Therefore, conductors and lead wires of the induction coil must have a large sectional area so that it is difficult to vertically move the induction coil 2. Although connected to pipes for cooling water, in contrast, the upper and lower crucibles 11 and 12 can be moved more easily by far than the induction coil 2. Also in an actual apparatus, consequently, it is configured so that the induction coil 2 is fixed and the lower crucible 2 is movable.
The position at which the upper crucible 11 is made contact with the lower crucible 12 in FIG. 9 must adequately be set. As described above, the induction coil 2 must allow a very large current to pass through, and realize a large ampere turn in order to produce a magnetic field of a given strength. Therefore, the dimension of the coil in the axial direction is requested to be as large as possible. When the height of the induction coil 2 is constant, the upper and lower crucibles of the crucible 1 make contact with each other at a position which is higher than the lower face of the induction coil 2, with the result that the lower portion of the induction coil 2 downward projects from the upper crucible 11. Under this state, as shown in FIG. 8, the downward movement of the lower crucible 12 causes a portion of the solidified portion 52 not to be in contact with the inner face of the crucible 1, and there may arise a phenomenon that a portion near the upper crucible 11 is again melted by magnetic fluxes entering from the outside and then solidified. This produces a problem in that the growth of the material to be melted 5 is obstructed. Furthermore, another problem is produced in that the portion of the induction coil 2 projecting from the upper crucible 11 is exposed to radiation heat of the hot material to be melted and the temperature of the portion is raised, whereby the deterioration of the insulating material is accelerated to shorten the life period. In order to prevent these problems from arising, the induction coil 2 must be located in such a manner that its lower face is set to be higher than the lower face of the upper crucible 11. This means that the plane in which the upper and lower crucibles 11 and 12 makes contact with each other must be located at a lower position. Under the state where the material to be melted 5 floats as shown in FIG. 9, the plane in which the upper and lower crucibles 11 and 12 makes contact with each other is located in the vicinity of the levitating material to be melted 5. As a result, magnetic fluxes entering through the plane in which the upper and lower crucibles 11 and 12 makes contact with each other affect the material to be melted 5 so that the portion of the material to be melted 5 in the vicinity of the plane is depressed by an electromagnetic force, thereby producing a problem in that the material to be melted 5 has an unstable shape or, for example, the material to be melted is deformed into a guitar-like shape. In fact, it is difficult in some cases to adequately set a positional relationship between the lower crucible 12 and the induction coil 2 in which all the above-discussed problems are prevented from arising.
In the case where the material to be melted 5 is titanium or zirconium as described above, when melted in the air, these materials are contaminated with impurities or an oxide film is formed, because they have a particularly high activity, thereby producing a problem in that the purity is lowered. When such a material is to be melted, therefore, employed is a system in which a levitating and melting apparatus is placed in a vacuum vessel and the melting process is conducted in a vacuum. As described above, in order to intensively cool the crucible 1 and the induction coil 2, the levitating and melting apparatus must be connected with the outside through the pipes for cooling water, the lead wires for supplying a current to the induction coil 2, etc. These connections must be conducted with passing through the vacuum vessel, thereby producing a further problem in that the configuration is complex and the cost of the apparatus is high.